1. Field of the Invention
The invention concerns a method and a control sequence determination device to determine a magnetic resonance system activation sequence for a follow-up measurement (following a first measurement of a selected image region of an examination subject) for at least a portion of the selected image region of the examination subject, the activation sequence including a multichannel pulse train with multiple, individual RF pulse trains to be emitted in parallel via different, independent radio-frequency transmission channels of a transmission device, wherein a multichannel pulse train is determined in order to achieve a defined, local target magnetization distribution upon emission of the calculated multichannel pulse train MP.
Moreover, the invention concerns a magnetic resonance system with a transmission device with a plurality of independent radio-frequency transmission channels, and a control device which is designed in order to emit a multichannel pulse train with multiple, parallel, individual RF pulse trains via the different radio-frequency transmission channels for implementation of a desired measurement based on a predetermined activation sequence.
2. Description of the Prior Art
In a magnetic resonance system, the body to be examined is typically exposed (with a basic field magnet system) to a relatively high basic field magnet field (is known as the B0 field) of 1.5 Tesla, 3 Tesla or 7 Tesla, for example. A magnetic field gradient is additionally applied with the aid of a gradient system. By means of suitable antenna devices, radio-frequency excitation signals (RF signals) are then emitted via a radio-frequency transmission system, causing the nuclear spins of specific atoms to be excited to resonance by this radio-frequency field and tilted by a defined flip angle relative to the magnetic field lines of the basic magnetic field. The radio-frequency magnetic field is also designated as a B1 field. This radio-frequency excitation or the resulting flip angle distribution is designated as a nuclear magnetization (or just “magnetization”) in the following. Upon relaxation of the nuclear spins, radio-frequency signals—known as magnetic resonance signals—are radiated and are received by means of suitable reception antennas and then are processed further. Finally, the desired image data can be reconstructed from the raw data acquired in such a manner. The emission of the radio-frequency signals for nuclear magnetic resonance magnetization for the most part takes place by means of what is known as a “whole body coil” or “body coil”. A typical design for this is a cage antenna (birdcage antenna) that has multiple transmission rods arranged parallel to the longitudinal axis and around a patient space of the scanner in which the patient is located during the examination. The antenna rods are connected with one another in an annular, capacitive fashion at their ends.
It has previously been typical to operate whole-body antennas in a “homogeneous mode”, for example a “CP mode”. For this purpose, a single, temporal RF signal is provided to all components of the transmission antenna, for example all transmission rods of a birdcage antenna. The transmission of the pulses to the individual components may take place with a phase offset, with a shift adapted to the geometry of the transmission coil. For example, in the case of a birdcage antenna with 16 rods, the rods can respectively be activated with the same RF signal with a phase shift offset of 22.5°.
For newer magnetic resonance systems, it has by now become possible to allocate individual RF signals, adapted to the imaging, to the individual transmission channels (which, for example, are associated with the individual rods of a birdcage antenna). For this purpose, a multichannel pulse train is emitted that, as described above, includes multiple individual radio-frequency pulse trains that can be emitted in parallel via the different, independent radio-frequency transmission channels. Due to the parallel emission of the individual pulses and as a “pTX pulse”, such a multichannel pulse train can be used as an excitation pulse, refocusing pulse and/or inversion pulse. The previous homogeneous excitation can thereby be replaced with an excitation of (in principle) arbitrary shape in the measurement space, and consequently also in the patient.
Such multichannel pulse trains are typically generated in advance for a defined, planned measurement. For this purpose, the individual RF pulse trains—i.e. the RF trajectories—are determined in an RF pulse optimization method for the individual transmission channels over time, depending on a “k-space gradient trajectory”. The “transmission k-space gradient trajectory” (in the following abbreviated only as “k-space gradient trajectory” or “gradient trajectory”) is the locations in k-space that are occupied at defined times by adjustment of the individual gradients. K-space is the positional frequency space, and the gradient trajectory in k-space describes the path along which points in k-space are temporally traversed upon emission of an RF pulse, or the parallel pulses, by appropriate switching of the gradient pulses. At which spatial frequencies specific RF energy magnitudes are caused to occur can thus be determined by adjusting the gradient trajectory in k-space, i.e. by adjusting the matching gradient trajectory applied parallel to the multichannel pulse train. In the definition of a gradient trajectory it is to be noted that the relevant regions in k-space must also be traversed. For example, if an area sharply delimited in position space—a rectangle or oval, for example—should be excited, k-space should also be well covered in its outer boundary region. In contrast to this, if only a fuzzy boundary is desired, a coverage in the middle k-space region is sufficient.
Moreover, the user provides a local target magnetization distribution—for example a desired flip angle distribution—for the planning of the RF pulse sequence.
The matching RF pulse sequence for the individual channels is then calculated with a suitable optimization program so that the local target magnetization distribution is achieved. For example, a method to develop such multichannel pulse trains in parallel excitation methods is described by W. Grishom et al.: “Spatial Domain Method for the Design of RF Pulses in Multicoil Parallel Excitation”, Mag. Res. Med. 56, 620-629, 2006.
For a defined measurement, the different multichannel pulse trains (that are to be emitted via the different transmission channels of the transmission device), the gradient pulse train (with matching x-, y- and z-gradient pulses) that is to be emitted in a coordinated manner for this purpose, and additional control specifications, are defined in a set of instructions or commands known as a measurement protocol which is created in advance and can be retrieved (for example from a memory) for a defined measurement and modified on site by the operator if necessary. While the measurement (data acquisition) takes place, the control of the magnetic resonance system ensues wholly automatically based on this measurement protocol, with the control device of the magnetic resonance system reading out the commands from the measurement protocol and executing them.
MR imaging represents an important measurement method in diagnostics and therapy in clinical practice. Particularly in therapy for oncological patients, process controls are to be implemented before, during and after therapy involving tissue-destroying methods. For example, tumor tissue and its dimensions can be monitored in the therapy. However, a reliable comparability of the relevant tissue dimensions detected by measurements that are separated in time is not possible without additional measures. The reproducibility of the slice positions and the variation of the tissue itself are a hindrance to reliable comparability between the measurement series.